Primates: The Opposable Thumb

Excerpt from Wikipedia, accessed 2/7/08, with comments by Suzanne Kemmer

Origin of the thumb

The evolution of the opposable or prehensile thumb is usually
associated with Homo habilis, the forerunner of Homo sapiens.[2][3][4]
This, however, is the suggested result of evolution from Homo erectus
(around 1 MYA) via a series of intermediate anthropoid stages, and is
therefore a much more complicated link.

The most important factors leading to the habile hand (and its thumb)
are:

the freeing of the hands from their walking requirements -
still so crucial for apes today, as they have hands for feet, which
in its turn was one of the consequences of the gradual
pithecanthropoid and anthropoid adoption of the erect bipedal walking
gait

[SK comment: the above is poorly written. I
believe the writer means that apes, unlike us, still crucially depend
on their hands (specifically knuckles) for walking; and the freeing of
the hands was one of the consequences of the adoption by
pithecanthropoid and anthropoid primates of erect bipedal
walking. This statement says that upright bipedal walking in hominoids
allowed the hands to be freed for other things such as tool/weapon
use.]

the simultaneous development of a larger anthropoid brain in the
later stages.

It is possible, though, that a more likely scenario may be that the
specialized, precision gripping hand (equipped with opposable thumb)
of Homo habilis preceded walking, with the specialized adaptation of
the spine, pelvis and lower extremities proceeding
[author presumably
means "preceding"- S.K.] a more advanced
hand. And, it is logical that a conservative, highly functional
adaptation be followed by a series of more complex ones that
complement it. With Homo habilis an advanced grasping-capable hand was
accompanied by facultative bipedalism, possibly implying, assuming a
co-opted evolutionary relationship exists, that the later [ = "latter"
-S.K.] resulted
from the former as obligate bipedalism was yet to follow.[5] Walking
may have been a byproduct of busy hands and not vice versa.

[The author of the above paragraph suggests that an advanced hand
structure with an opposable thumb could have preceded bipedal walking in the
hominid line. Although this hypothesis seems to be treated as an
opposing one to the hypothesis above, I don't see any
contradiction. It is entirely possible that first came the fancy hand, then upright bipedal walking; then
the freed up hands could be used for holding, carrying, and throwing.
Maybe the hands did some further evolving after bipedal gait, too. S.K. ]

Importance of the opposable thumb

The thumb, unlike other fingers, is opposable, in that it is the only
digit on the human hand which is able to oppose or turn back against
the other four fingers, and thus enables the hand to refine its grip
to hold objects which it would be unable to do otherwise. The
opposable thumb has helped the human species develop more accurate
fine motor skills. It is also thought to have directly led to the
development of tools, not just in humans or their evolutionary
ancestors, but other primates as well.[6][7] The thumb, in conjunction
with the other fingers make humans and other species with similar
hands some of the most dexterous in the world.[8]

Other animals with thumbs

[S.K. comment: I said in class that only humans
had opposable thumbs, because many works on hominids speak of the
opposable thumb as an exclusive innovation of the hominid line. (For
example see Footnote 3 below.) However, I see now that some qualify
the statement and say humans are the only creatures with a fully
opposable thumb. How exactly you can have a semi-opposable thumb
is not clear to me. But the animals below do have something
functionally and structurally analogous to our thumbs, either on hands
or feet or both. I do not know what exact structural and functional
differences there are between humans' "fully opposable" thumb and the
thumb-type digits of other primates. The pictures of chimp and
gorilla hands and feet that I put in the Resources page on Owlspace
(folder: Great apes) are useful for seeing the similarities, and
perhaps the gross shape differences in hands and feet between the pongidae and
to genus Homo, but the pictures alone do not allow us to derive
a full understanding of what are the really important structural and
functional innovations in our hands.]

Many animals, primates and others, also have some kind of opposable
thumb or toe:

* Bornean Orangutan - opposable thumbs on all four hands. The
interdigital grip gives them the ability to pick fruit.
* Gorillas - opposable on all four hands.
* Chimpanzees have opposable thumbs on all four hands.
* Lesser Apes have opposable thumbs on all four hands.
* Old World Monkeys, with some exceptions, such as the genera,
Piliocolobus and Colobus.
* Cebids (New World primates of Central and South America) - some
have opposable thumbs
* Koala - opposable toe on each foot, plus two opposable digits on
each hand
* Opossum - opposable thumb on rear feet
* Giant Panda - Panda paws have five clawed fingers plus an extra
bone that works like an opposable thumb. This "thumb" is not
really a finger (like the human thumb is), but an extra-long
sesamoid bone that works like a thumb.
* Troodon - a birdlike dinosaur with partially opposable thumbs.
* Raccoon - a common mammal with thumbs, which are not opposables.